Several groups of motoneurons and sensory neurons have been physiologically identified in the neural control of feeding in the opisthobranch mollusc Navanax inermis. The anatomy of individual buccal neurons is being pursued by intracellular dye injections, using Lucifer Yellow, cobalt acetate, and HRP. Serial reconstruction of the buccal ganglia from thick sections can be utilized to identify these same neurons by their position, disposition of neurites, and somatic size and shape. The ultrastructure of portions of identified neurons can be determined from this section series through remounted thick section, or by using an intracellular marker such as HRP. Physiological and morphological studies of motoneurons have localized probable sites of electrical coupling at the distal edges of the buccal ganglia. Freeze fracture studies and conventional EM studies are being combined to explore the morphological basis for the electrical coupling between these buccal neurons. Gap junctions between isolated pairs of early blastomeres from the teleost, Fundulus, will be utilized to estimate the channel conductivity per structural unit in the junction. Junctional conductance (gj) is measured physiologically; then the same pair of cells is serially sectioned to reconstruct the size of the gap junction. The density of channels can be determined from freeze fracture studies. The conductance of a single channel is the ratio of the gj to the number of channels in the reconstructed junction, assuming that all channels have uniform properties.